By brush do you mean having brushes as opposed to being brushless or do you mean it is made by Brush ( of louborough if British)
Either way I think you have a very old machine if it has a commutator.
The most likely thing is that you have a cross field self regulating machine and if so it is rather complex to understand if you have no experience of these things.
What I think you will find it that it has two sets of brushes on the commutator, one conventional set feeding a shunt field winding ( probably with a series resistance for voltage adjustment. Another set of brushes set at an angle to these will be connected to a very low resistance field winding with thick wires. These are the cross axis brushes and are effectively shorted together. The cross field current is changed by the armature reaction of the ac load winding and the current in this low resistance field bucks or boosts the main shunt field to maintains constant ac volts with load.
They were very reliable machines and worked well with fairly resistive loads, the regulation was poor with low power factor loads such as welders and the motor starting ability was not the best.
If you have reason to mess with the brush gear be very careful, the brush position is very critical and in fact the regulation is altered by moving the cross field brushes.
One way gives a drooping voltage with load but if you go the other way it will rise, the correct position is where the volts remain constant.
These things are critical on commutator condition and brush bedding.
They are very similar to an Amplidyne in operation and share many of the brush and commutator quirks of the amplidyne.
I am not sure if you can get any drawings on the internet, there were many variants of this design by Mawdsley, Electric Construction, Arthur Lyon (Newage ) and others. If yours is Brush Electric then it may have been called a square path or square field machine if it is the variety with a square field frame rather than a round one.
To add to the confusion it may also have another series field that was used to motor the thing to start the engine, this field has no other purpose.
I hope this helps but these have been out of production many years and few people will understand them.
Flux